Yeah... not a safe situation. My sister's husband had his windshield shattered by a load of snow and ice coming off an overpass just a dozen feet up so I can imagine the dammage done when a larger chunk has the time to hit terminal velocity before impact (he's okay... was shaken up by the incident though.)
Chubbster, why are you 'bumping' this (and only two minutes after the initial post)? Do you think that people who may be driving on the M4 towards the bridges are logged on to Arrse? I'm sure the radios, Highway Dept and the relevant local authorities are disseminating the information suitably without needing you to 'give them a hand'. Do us all a favour and pop across to the bridges. Stand under a rather large lump of ice and let us know when it falls? Thanks.